1. Field of the Invention
The field of this invention relates to radio frequency transmitters having power combiners. The invention is applicable to, but not limited to, efficient power combining techniques.
2. Background of the Invention
In the field of complementary metal-oxide-semiconductor (CMOS) power amplifiers, there is a constant demand by consumers for wireless systems that are low cost, efficient and reliable. High level integration has been shown in practice to be an effective way to reduce cost and achieve compactness for high volume applications. Currently, almost all power amplifiers are manufactured with III-V compound semiconductors, as this type of device has high output power and high power efficiency, which are desired attributes for many power amplifier applications. It is currently very difficult to achieve these specifications using CMOS technology. However, III-V technologies have high manufacturing costs, as well as being unable to provide a complete system-on-chip (SoC) solution.
Recently, CMOS power amplifiers coupled to power devices have become more attractive, partly because the power device technology and processing has matured and become lower cost. However, in implementing CMOS power amplifiers and integrating with power devices, there is still a need to improve the coupling, efficiency, and termination efficiency of these devices. Such devices are often used in radio frequency transmitters, for either base station or subscriber unit devices.
Referring first to FIG. 1, a simplified integrated radio frequency power amplifier (RF PA) system 100 comprises integrated circuit 102 and integrated power devices 150, 170. Integrated circuit 102 comprises power inputs 104 operably coupled to isolation transformers 106. The outputs of the isolation transformers 106 are operably coupled to respective driver stages 108 wherein the driver stages 108 provide amplified radio frequency signals to inputs of power stages 110, e.g. power amplifiers. In this case, the respective power stages 110 are operably coupled, via a plurality of bond wires 112, to power combiner devices 150, 170. Generally, the bond wires 112 are kept as short as possible in order to minimise radio frequency signal losses, as well as to reduce the area taken up by the integrated RF PA system 100, which is especially useful when using CMOS devices.
Due to the closeness of many of the inductive components within the integrated RF PA system 100, there are a number of potentially problematic effects. For example, there can be mutual coupling between the bond wires 112 and the power combiner elements 152, 172 of the integrated power combiner devices 150, 170. There may also be mutual coupling between the power combiner elements 152 and 172 themselves within the integrated power combiner devices 150, 170. These can be caused by an imbalanced impedance transformation and poor common-mode harmonic suppression within the integrated power combiner devices 150, 170. This problem is usually solved by increasing the spacing between the integrated power combiner devices 150, 170. However, this is not always a viable option for devices when trying to reduce the overall size of the integrated system 100.
Another common problem is centre-tap harmonic bounce at the connectors 114 between the IC 102 and integrated power combiner devices 150, 170. This, again, can be caused by an imbalanced impedance transformation and poor common-mode harmonic suppression.
Thus, a need exists for an improved coupling regime between RF IC 102 and integrated power combiner devices 150, 170.
Referring now to FIG. 2, there is illustrated a simplified known power combiner 200 that may be utilised in FIG. 1, for example, within the integrated power devices 150, 170. Simplified known power combiner 200 comprises two primary windings 201, 203, operably coupled to input connectors 205, and further operably coupled to each other via centre tap connector 207, and secondary winding 209, which is somewhat isolated from the two primary windings 201, 203.
A schematic diagram of the layout of such a simplified known power combiner 200 is illustrated in 250, which comprises a series of primary windings 260, 270, 280, 290 that are isolated from each other, and a ‘figure 8’ layout for the secondary winding 254. Each isolated primary winding 260, 270, 280, 290 comprises an interleaved structure 252 situated above and below the secondary winding 254, which reduces losses and enhances magnetic coupling. Due to the ‘figure 8’ layout for the secondary winding 254, the secondary winding is somewhat immune to common mode disturbances because the incoming magnetic flux induces currents of opposite direction across each ‘figure 8’ section. In this case, the interleaved structure 252 of the primary windings 260, 270, 280, 290 are operably coupled to each other at supply modules 256. In this case, the ‘figure 8’ secondary winding 254 is operably inductively coupled to all of the primary windings, 260, 270, 280, 290.
An illustration of current flows in schematic diagram 250 are illustrated in 295, which comprises a section of secondary winding 254, and primary winding 260. In this illustration, the interleaved structure 252 of primary winding 260 has been offset to illustrate current flows in each part of the interleaved structure 252. The interleaved structure 252 of the primary winding 260 facilitates coupling above and below the section of the secondary winding 254.
From the illustration of current flows in 295, it should be clear that the series of primary windings 260, 270, 280, 290 are not in ‘figure-8’ structures, but a series of non-figure-8’ shaped primaries that are isolated from each other. Further, the resultant current flow in secondary winding 254 runs perpendicular to the current in the series of primary windings 260, 270, 280, 290.
A potential problem with this structure is that the primary windings 260, 270, 280, 290 will still couple to each other, which would result in an imbalanced impedance transformation and poor common-mode harmonic suppression at each differential port. In some severe cases, an imbalanced impedance transformation may affect the output power and efficiency of this structure.
It is known that mutual coupling can be a serious problem for certain devices, for example radio frequency transceivers, as the effect of mutual coupling can change antenna array radiation patterns and alter matching characteristics of antenna elements. Thus, a need exists for an improved power combiner for an IC.
In some cases, it may be desirable to suppress the common-mode power of a power combiner. In the field of this invention, this is achieved by terminating at the fundamental frequency of a power amplifier.
Thus, a need exists for an improved power combiner, a transformer and a termination arrangement for a radio frequency transmitter, for example a power combining IC.